The present invention relates to a method for operating a drawing press which produces a drawn part during each working cycle, one blank being inserted into the drawing tool of the drawing press, this tool including a die, a punch and a blank holder, the blank being clamped in by the blank holder at an edge with a specific clamping force, and the drawn part being subsequently drawn between the die and the punch.
Such a method is disclosed for example in an article by F.-J. Neff, "CNC and DNC operation in hydraulic presses" in the publication Werkstatt und Betrieb (Workshop and Plant), 119 (1986) 11, pages 947 to 949.
In hand-fed drawing presses it is customary practice in pressing plants to correct the drawing process, which occurs in a timed sequence, on the basis of a continuous visual inspection of the drawn parts by the operating personnel and of an individual manual intervention in the adjustment of the blank-holding force. This is therefore a case of an adjustment process in which the human being is included as an essential, process-determining element. Apart from the monotony associated with this and the required constant attentiveness and responsibility of the operating personnel, drawn part errors resulting from an inaccurate or incorrect adjustment of the blank-holding force are often not promptly detected so that despite a constant monitoring of the drawing processes, faulty drawn parts leave the drawing press and adversely affect the productivity of the drawing press. In automatically fed presses or in pressing trains, only random sample-like visual inspection is carried out so that, particularly in modem pressing plants, there is a greater risk of rejected parts than in plants which still have complete manual operation.
In the aforementioned article by Neff, the author reports on a system for automatic quality inspection in pressing plants with appropriately developed hardware and software for a largely optimized press operation. Displacement sensors and pressure sensors for slides and die cushions are integrated into the presses. As a result, the stroke/slide force curve for each individual workpiece can be measured and also displayed with a monitor. This actual-value curve can be compared for each individual workpiece with a workpiece-specific reference curve. At the start of production, the reference curve is produced or empirically determined for a specific workpiece to be manufactured and the data are stored; in fact, for example the stroke/slide force curve of the first fault-free drawn part can be used as a reference curve. By means of the prescribed procedure and other measures not mentioned here, rapid refitting of a press to other workpieces and a monitored, i.e. failure free press operation, or press operation in which an alarm is automatically given in the event of a failure, is ensured. It is mentioned that rejected parte during press operation as a result of tool wear can arise as a result of quality changes on the workpiece with respect to dimensions or material or as a result of quality of the lubrication. By means of a repeated comparison, in a timed sequence, of the variation of the workpiece individual stroke/slide force curve with the reference curve, rejected parts can be detected automatically and early. In the event of a tolerance range which "accompanies" the reference curve being exceeded or undershot, a fault is reported and the machine is deactivated so that, if appropriate, intervention by personnel can occur. The press itself which is monitored in such a way obviously operates, at least until the next failure, with a constant setting of all process parameters.
In another article, by D. Bauer, G. Gucker and R. Thor, "Computer-Supported Blank-Holding Pressure Optimizes Deep Drawing" in the publication Bleche-BanderRohre (Sheet Metal, Strip Metal, Pipes) 5-1990, pages 50 to 54, the authors initially point out that for the deep drawing of fault-free pans it is necessary for the blank-holding force not to be allowed to undershoot a specific minimum value which changes as a function of stroke and not to exceed a specific maximum value which also changes as a function of stroke, the curves for the minimum values and maximum values behaving in a workpiece-dependent fashion. Excessively high blank holding forces lead to fractures on the drawn part, whereas a blank holder which is pressed on too weakly allows folds to arise. The article recommends deviating from the previously widespread variation of the blank holding force which had a more or less high degree of constancy and using a variation of the blank-holding force against the press stroke which is optimized in dependence on the type of workpiece, it being possible for such a non-constant blank-holding force variation to be made up from several sections of a constant and/or a linearly rising or descending course or from a functionally stipulated course. The desired-value variation for the blank-holding force can be optimized in various aspects according to the cited publication and, depending on the optimization objective, possibly also has a different appearance. For example, the blank-holding force variation can also be optimized with respect to the maximum drawn part quality, in which case it is also possible here again for different considerations, depending on the type of workpiece, to be emphasized, for example freedom from fractures or folds or avoidance of shrink marks. Instead, when optimizing the blank-holding force variation, the design of the drawing process can also be more significant, for example the increase in the acceptable drawing depth with the objective of possibly being able to omit a drawing stage or save on sheet metal or achieve a greater strength of the drawn part. Tribological considerations can also be included in the optimization of the variation of the blank-holding force. The optimized blank-holding force variation, once it has been ascertained for a specific workpiece, is then followed up in a closed-loop controlled fashion during each pressing cycle, the ascertained desired-value curve, with the exception of occasional, subsequent manual improvements, being, however, uniformly maintained. Despite the use of a variation of the blank-holding force which is optimized to this extent and a corresponding closed-loop control in accordance with this variation, the aforesaid article does not go into detail on an automatic detection of errors on the dram part.
An object of the invention is to improve the method of the genetic type to the extent that, in the case of non-optimum setting of the process parameters or in the case of a failure which is caused for example by quality changes or lubrication changes on the part of the workpiece, the latter can be detected automatically and early, i.e. while the drawn part is still in the working space of the press, and a suitable correction of the set value of the clamping force of the blank holder can become effective immediately, i.e. for the next workpiece and can also be performed automatically.
This and other objects are achieved by the present invention which provides a method for operating a drawing press which produces a drawn part during each working cycle, one blank being inserted into the drawing tool of the drawing press, this tool including a die, a punch and a blank holder, the blank being clamped in by the blank holder at an edge with a specific clamping force and the drawn part being subsequently drawn between the die and the punch. The method comprises, before starting up production of drawn parts of a specific type, determining and storing: an optimum drawing force variation, dependent on at least one of time and pressing stroke, of a drawing punch force exerted on the drawn part during the drawing process; an upward deviation from the optimum drawing force variation which is acceptable without risking production of fractures; and a downward deviation from this optimum drawing force variation which is acceptable without risking production of folds, such that for the specific type of drawn part to be drawn, data for a desired-value drawing force range which is dependent on at least one of time and pressing stroke is stored. The drawing force must vary within the desired-value drawing force range in order to expect acceptable drawn parts that are fracture-free and fold-free. During each working cycle during production of drawn parts of the specific type, an actual-value drawing force variation is measured that is dependent on at least one of the time and the pressing stroke, of the drawing force exerted on the drawn part during the drawing process. The quality of the drawn part is automatically monitored during each working cycle with respect to the fractures and folds by comparing the data of the actual-value drawing force range, this comparing including determining whether the actual-value drawing force variation: varies within the desired-value drawing force range during the entire drawing path; exceeded the desired-value drawing force range to indicate fractures; or undershot the desired-value drawing force range to indicate folds. The clamping force which can be set at the blank holder is automatically optimized, the clamping force for the following working cycle being changed or maintained uniformly as a function of the detected drawn part quality of a drawn pan drawn in a preceding working cycle. This automatic optimizing includes: lowering the clamping force for the following working cycle with respect to the value of the clamping force set in the preceding working cycle which resulted in a fractured drawn part quality of the previously drawn part; uniformly maintaining the clamping force in the following cycle when the previously drawn part is fault-free and is of acceptable drawn part quality; and increasing the clamping force for the following working cycle with respect to the value of the clamping force set in the preceding work cycle which resulted in folded drawn part quality of the previously drawn part. At least one of the time and the degree of exceeding or undershooting within the working cycle of the desired-value drawing force range by the actual-value drawing force variation is detected, the at least one of time and the degree of exceeding and undershooting being hereinafter referred to as the damage signal. The clamping force of the blank holder is changed to a greater extent the earlier the damage signal occurs and the stronger the damage signal is, in comparison to when a damage signal occurs late or a weaker damage signal occurs.
According to the prior art, before starting up production for each type of a part to be drawn a range, dependent on time or press stroke, of the drawing punch force exerted on the part during the drawing process, the "desired drawing force range", is determined and the data are stored, within which range the drawing punch force must vary in order to be able to expect fracture-free and fold-free, that is to say "acceptable" drawn parts. Therefore, during each press stroke the actual-value drawing force variation which occurs over time can subsequently be measured and it is possible to monitor whether this variation stays within the desired-value drawing force range and whether it has exceeded (fractures) or undershot (folds) the desired-value drawing force range.
According to the invention, this possibility of an automatic fault detection on the drawn part with respect to the "fracturing" and "folding" types of fault is utilized during the drawing process itself in order to make automatic corrective interventions so that the press can continue to operate in the event of failures and, at most, one faulty part or, in the case of serious failures, possibly two faulty parts are pressed and subsequently acceptable parts are produced again. By means of the automatic fault detection, the method of process optimization which was previously operated, that is to say controlled, manually and under the inspection of humans, becomes a control process which proceeds automatically and in a closed cycle. According to the invention, during the automatic fault detection and technical control process adaptation, the time and/or the degree of the damage signal is detected within the respective working cycle, in which case the clamping force of the blank-holder is changed to a greater extent when a damage signal occurs early or when a stronger damage signal occurs than when a damage signal occurs later or a weaker damage signal occurs.
Expedient embodiments of the invention provide automatic detection of fluctuations of process parameters and/or of quality fluctuations of the semifinished product, which fluctuations require in each case a corresponding adaption of the blank-holding force in order to achieve optimum process control. Fluctuations of this kind are caused in particular by changes in
the material strength of the sheet bars, PA1 the thickness of the sheet metal, PA1 the roughness of the surface of the sheet bars, PA1 the thickness of the lubrication film and PA1 the viscosity of the lubricant.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.